1. Field of the Invention
The present invention generally relates to inkjet printers and, more particularly, to nozzle plates and other components for the printheads of inkjet printers.
2. State of the Art
FIG. 1 shows an example of a conventional printhead for an inkjet printer. The printhead includes a substrate 11, an intermediate layer 14, and an nozzle plate 12. As further shown in the drawing, a nozzle orifice 13 is formed in plate 12 and a vaporization cavity 15 is formed in substrate 11. For convenience of illustration, the drawing shows only one of the orifices 13 and only one of the vaporization cavities 15; however, a complete inkjet printhead includes an array of circular orifices, each of which is paired with a vaporization cavity. Also, a complete inkjet printhead includes means that connect a number of vaporization cavities to a single ink supply reservoir.
As further shown in FIG. 1, a heater resistor 16 of the thin-film type is mounted on substrate 11 and is positioned generally centrally within vaporization cavity 15 such that the heater resistor can be seen when the vaporization cavity is viewed from above. In practice, such heater resistors can be formed on a silicon or glass substrate, for example, by sputtering or vapor deposition techniques. Conventional printheads for inkjet printers include one such heater resistor in each vaporization cavity and the heater resistors are connected in an electrical network for selective activation.
In operation of a inkjet printhead such as shown in FIG. 1, pulses of electrical energy are directed to selected ones of the heater resistors 16. When a particular heater resistor receives a pulse, it rapidly converts the electrical energy to heat which, in turn, causes any ink immediately adjacent to the heater resistor to form an ink vapor bubble. As an ink vapor bubble expands, it ejects a droplet of ink from the orifice in the nozzle plate above the energized heater resistor. To illustrate such action, FIG. 1 shows an ink vapor bubble 17 and an ink droplet 19.
By appropriate selection of the sequence for energizing the heater resistors in an inkjet printhead such as shown in FIG. 1, ejected ink droplets can be caused to form patterns on a paper sheet or other suitable recording medium. For example, a pattern of heater resistors can be energized such that the ejected ink drops form images that depict alphanumeric characters.
For inkjet printers, print quality depends upon the physical characteristics of the nozzles in a printhead. For example, the geometry of the orifice nozzles in a printhead affects the size, trajectory, and speed of ink drop ejection. In addition, the geometry of orifice nozzles in a printhead can affect the flow of ink supplied to vaporization chambers and, in some instances, can affect the manner in which ink is ejected from adjacent nozzles.
Nozzle plates for inkjet printheads often are formed of nickel and are fabricated by lithographic electroforming processes. One example of a suitable lithographic electroforming processes is described in U.S. Pat. No. 4,773,971. In such processes, the orifices in a nozzle plate are formed by overplating nickel around pillars of photoresist.
Such electroforming processes for forming nozzle plates for inkjet printheads have several shortcomings. One shortcoming is that the processes require delicate balancing of parameters such as photoresist and plating thicknesses, pillar diameters, and overplating ratios. Another shortcoming is that the resulting nozzle plates usually are brittle and easily cracked. Still another shortcoming is that such electroforming processes inherently limit design choices for nozzle shapes and sizes.
When using electroformed nozzle plates and other components in printheads for inkjet printers, corrosion can be a problem. Generally speaking, corrosion resistance of such nozzle plates depends upon two parameters: ink chemistry and the formation of a hydrated oxide layer on the electroplated nickel surface of an nozzle plate. Without a hydrated oxide layer, nickel may corrode in the presence of inks, particularly water-based inks such as are commonly used in inkjet printers. Although corrosion of nozzle plates ca be minimized by coating the plates with gold, such plating is costly.
Yet another shortcoming of electroformed nozzle plates for inkjet printheads is that the completed printheads have a tendency to delaminate during use. Usually, delamination begins with the formation of small gaps between a nozzle plate and its substrate. The gaps are often caused by differences in thermal expansion coefficients of a nozzle plate and its substrate. Delamination can be exacerbated by ink interaction with printhead materials. For instance, the materials in an inkjet printhead may swell after prolonged exposure to water-based inks, thereby changing the shape of the printhead nozzles.
Even partial delamination of a nozzle plate of an inkjet printhead can be problematical. Partial delamination can, for example, reduce the velocity of ejected ink drops. Also, partial delamination can create accumulation sites for air bubbles that interfere with ink drop ejection. Moreover, partial delamination of a nozzle plate usually causes decreased and/or highly irregular ink drop ejection velocities.